In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium to form characters or images. The ink-jet printing methods have become rapidly spread because of various advantages such as easiness of full coloration, low cost, capability of using a plain paper as the recording medium, non-contact with printed characters or images, etc. In recent years, in order to impart good weathering resistance and good water resistance to printed materials, an ink containing a pigment as a colorant has been extensively used.
Further, for the purpose of stably compounding a pigment in a water-based ink, colored fine particles prepared by encapsulating the pigment with a polymer have been developed.
For example, JP 2003-306611A discloses a process for producing a microencapsulated pigment capable of producing an ink for ink-jet printing which can satisfy all of requirements (1) to (6) including (1) excellent dispersion stability; (2) excellent ejection stability from a print head; (3) capability of procuring printed materials having images with excellent fastness; (4) capability of procuring printed materials having images with high optical density; (5) capability of procuring printed materials having images with excellent scratch resistance; and (6) capability of procuring printed materials having characters or images with high color developability which hardly suffer from ink bleeding even when using a plain paper as a recording medium, in which the pigment particles having a hydrophilic group on a surface thereof are coated with a polymer by an emulsion polymerization method.
In addition, JP 2003-261605A discloses a sulfuric acid salt containing an alkylene group having 5 to 18 carbon atoms, or an alkoxymethylethylene group and an alkylene group having 2 to 4 carbon atoms, as a polymerizable surfactant capable of providing a polymer emulsion that has good stability during polymerization thereof or provides a polymer coating film having good properties such as good water resistance.